This invention relates to systems wherein a light transmitter/receiver operates in an environment including high-power radio-frequency radiation.
Light transmitter/receivers include light transmitters such as lasers and light receivers such as light-sensitive cameras and sensors. Light transmitter/receivers must sometimes operate in an environment that includes radiation at frequencies of about 0.9 GHz (gigahertz) and higher, and with the radiation at power levels exceeding about 1 milliwatt per square centimeter. Such a radiation environment is terned xe2x80x9chigh-power radio-frequency radiationxe2x80x9d herein. An example is a light-receiver camera that is positioned to observe wave soldering processes conducted within an industrial microwave oven. Another example is a light-transmitter laser that provides spot-heating in the same industrial microwave oven.
The high-power radio-frequency radiation may disrupt the operation of the light transmitter/receiver by interfering with its electronic processes. In some circumstances the high-power radio-frequency radiation may damage or even destroy the light transmitter/receiver. It is therefore necessary to shield and protect the light transmitter/receiver from the high-power radio-frequency radiation, but without substantially interfering with the operation of the light transmitter/receiver.
Windows are known that are transparent to light but reduce or prevent the penetration of radio-frequency radiation. Such windows may be placed between the environment of radio-frequency radiation and the light transmitter/receiver to shield and protect the light transmitter/receiver. Two basic types of windows are known. In one, a light-transparent window substrate has a grid of metallic lines extending over its transverse extent. The grid prevents intrusion of the radio-frequency radiation, while permitting passage of a portion of the incident light. The grid has the shortcoming that it casts a shadow, either onto the surface of a light receiver or into the beam of a light transmitter. The grid may itself be damaged by the incident light energy of a powerful light transmitter such as a laser. The other type of window has a coating of an electrically conductive material on the light-transparent window substrate. The coating inhibits the penetration of radio-frequency radiation through the window. Examples of known coatings include tin oxide and indium-tin-oxide. The coating casts no shadow, but it has the shortcoming that it is effective in blocking radio-frequency radiation of only relatively low power levels, such as up to about 0.1 milliwatt per square centimeter, far below the regime of the high-power radio-frequency radiation.
At the present time, those who use light transmitter/receivers in the environment of high-power radio-frequency radiation must accept the presence of shadows cast by metallic grids. In some applications, however, such shadows are unacceptable. There is a need for an improved approach to the shielding and protection of light transmitter/receivers that must operate in an environment of high-power radio-frequency radiation. The present invention fulfills this need, and further provides related advantages.
This invention provides a method and structure for shielding a light transmitter/receiver against high-power radio-frequency radiation. The structure is in the form of a window, through which light is transmitted from or to the light transmitter/receiver. The approach of the invention may be used where the radio-frequency radiation is of high power, and is not limited by the use of coatings such as tin oxide and indium-tin-oxide. No shadow is produced in the beam of the light transmitter/receiver. The window may be easily and inexpensively constructed.
In accordance with the invention, a light transmitter/receiver is shielded against high-power radio-frequency radiation having a frequency exceeding about 0.9 GHz (gigahertz) and a power exceeding about 1 milliwatt per square centimeter. The method includes providing a light transmitter/receiver, and providing a window. The window comprises a first light-transparent plate, and a second light-transparent plate spaced apart from the first light-transparent plate. The two light-transparent plates are desirably, but not necessarily, parallel. Their surfaces may be coated with anti-reflective coatings. A liquid fills the space between the first light-transparent plate and the second light-transparent plate. The liquid comprises water, preferably pure water, but aqueous solutions such as salt brines may also be used. A frame may be provided to support the first and second light-transparent plates, and to seal against leakage of the liquid. The window is positioned between the light transmitter/receiver and a source of radio-frequency radiation with the first light-transparent plate facing the source. Preferably, either the source of the high-power radio-frequency radiation or the light transmitter/receiver is placed into a housing, with the window being through a wall of the housing.
The light transmitter/receiver may be a light source-transmitter such as a laser, or a light receiver such as an electronic light sensor. The first light-transparent plate and the second light-transparent plate may be any light-transparent material, such as glass or plexiglass. Preferably, the two light-transparent plates have high transmittance and low attenuation at the wavelength(s) of the light, and are no thicker than necessary to contain the liquid. The liquid within the space between the two light-transparent plates may be stationary, or it may be flowed through the space.
The approach of the invention uses a water-filled window to protect the light transmitter/receiver from external high-power radio frequency radiation. The window is easily and inexpensively constructed, and has no noxious constituents. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.